The invention relates to a radar system of the frequency modulated continuous wave type (FM/CW), intended for range measuring, including a linearly frequency modulated microwave signal generator, a transmitter-receiver antenna, and deriving means for deriving a fractional signal of the transmitted wave called the local oscillation signal and for deriving a fractional signal of the received echo wave and for transferring the said two signals to the inputs of a mixer, in which the lengths of the transfer lines are adjusted so that at least the interference internal coupling signal due to the antenna reflection has, up to the mixer, the same propagation time as the local oscillation signal with the possibility of static adjustment of the phase between these two signals.
Radars with single antennas are used, in general, for measuring long ranges. The power is, in this case, sent in trains of waves, which enables independence from the internal interference echoes of the radar. The invention proposes to produce a radar with a single antenna which can measure short ranges, in the order of a few meters; it applies more particularly to radioaltimetric probes which can be used on board missiles and are capable of measuring ranges as small as 3 meters.
The main disadvantages of radar or radioaltimeter systems with two antennas, this being mentioned as a technical problem of the background of the invention, are as follows: the antennas cannot be integrated with the equipment, but must be fixed on the side of the carrying aircraft and interconnected with the equipment by means of transmission lines whose transmission delay must be calibrated, these lines being able to be the source of errors in the measurement of range. In addition, when the distance separating the antennas is not negligible compared with the distance from the ground or from the object to be detected, the geometry of the antenna system becomes imperfect, which again results in a source of errors at small ranges. Finally, it is desirable to simplify the antenna system for purposes of compactness, installation in the aircraft, and, in general, economy.
It is assumed hereafter that the radar transmits an asymmetrical saw-tooth waveform with a constant positive slope for a given range, these saw teeth, of duration T, being separated by (constant) level sections, i.e. these sawteeth are produced with a repetition period T.sub.r in which T.sub.r is greater than T, whatever T may be. Under these conditions, the general formula which expresses the actual functional mode of the radar is: ##EQU1## where: f.sub.b : subtractive beat frequency between the transmitted wave and the received echo wave;
.tau.: delay time between transmitted wave and received echo wave; PA0 .DELTA.F: frequency excursion of transmitted signals or wobulation frequency.
The actual range D has a linear relationship with the delay .tau., according to the formula: EQU .tau.=2D/c (2)
By combining the formulae (1) and (2) the following basic formula is obtained: ##EQU2## c being the speed of an electromagnetic wave in air. The useful signal is produced by a mixer which produces the subtractive beat between the local oscillation, sampled from the transmission, and the received waves. If V.sub.OL is the voltage of the local oscillator and V.sub.R is that of the received wave, the beat output voltage v.sub.b of the mixer is equal to: EQU v.sub.b =K.multidot.V.sub.OL .multidot.V.sub.R .multidot.sin(2.pi.f.sub.b t+.rho.) (4) where
where EQU .rho.=2 .pi..multidot..tau..multidot.F.sub.min
F.sub.min being the minimum frequency of the transmitter and K being the conversion coefficient of the mixer. In the case of a saw-tooth modulation, the signal generally used in radioaltimeters is constituted from a succession of sinusoidal trains, whose duration T is that of the saw teeth, separated by (constant) level sections T.sub.r -T. At each level section, the beat frequency v.sub.b of the mixer is fixed at the value: EQU V.sub.b =K.multidot.V.sub.OL .multidot.V.sub.R .multidot.sin.gamma.(5)
It will be noted that the period of the beat signal corresponds with the wobulation repetition period T.sub.R. Its frequency spectrum is therefore formed of harmonics of the frequency: EQU f.sub.R =1/T.sub.R
The spectrum of the beat signal can be calculated by Fourier transform from its temporal form. It represents the power density at the harmonics of the wobulation frequency f.sub.R. Starting from this calculation, it is possible to analyse the spectra by varying the different parameters and particularly by observing their evolution at short ranges. Among several possible options for the functioning of the radar, it is preferably chosen to adjust the beat frequency f.sub.b of the signals to the fixed tuning frequency f.sub.bo of the receiver. For the calculations, f.sub.bo is for example given the value 25 kHz. The analysis of the spectra enables the following phenomena to be observed:
The envelope of spectrum approximates to a sin x/x curve centered on the frequency f.sub.bo when the number n of periods of the sinusoid according to the above formula (4) is rather large, during the wobulation, and of the order of 10 or more, this number n being able to be expressed by: EQU n=f.sub.b .multidot.T (6)
or by EQU n=.tau..multidot..DELTA.F (7)
When the number n is equal to a few units only, the sin x/x curve is distorted and the maximum shifts towards the low frequencies; to a first approximation, the relative error in the measurement of the range D is equal to: ##EQU3## where: F.sub.M = the frequency of the maximum of the spectrum. In order to keep the relative error in D, within 10% it is necessary that the number N is greater than 3. In other words, this means that the waves must be received with a delay .tau. longer than 3/.DELTA.F (formula (7)), or if, formulae (2) and (7) are combined, the minimum range D.sub.min of the obstacle or the target must be equal to: ##EQU4##
Apart from the abovementioned disadvantages of two-antenna FM/CW radars, it is demonstrated that, on the other hand, two-antenna radars easily satisfy the accuracy and sensitivity conditions imposed in the range 4.2 to 4.4 GHz reserved for radioaltimeters. In particular a delay line, placed either in the transmission channel or in the reception channel, solves the problem of accuracy at lower altitudes, because it artificially increases the range of the target. In this case, the use of isolators reduces the internal leakages without attenuating the useful signal and it is thus possible to measure ranges from zero. With a single antenna, the problem of self-dazzle noise is much more difficult to solve: the radar receives, over the same channel, the wave reflected by the antenna and the useful wave coming, as an echo, from the target. The use of isolators and a delay line is therefore impossible without attenuating the useful signal.
The detection of close targets by a single antenna radar is therefore a problem which is intrinsically very difficult to solve. By way of indication, a frequency excursion .DELTA.F of 150 MHz limits the radar measurement to ranges greater than 3 m, when the above formula (9) is applied. To further decrease the minimum range, an increase in the value of .DELTA.F can be attempted but two difficulties then arise: on the one hand it is difficult to obtain a source which is linear in frequency over a wide frequency excursion, greater than 150 MHz. On the other hand, and this is the basic problem, the phase of the coefficient of reflection of the antenna is not linear over a wide pass band. In the short term, however, it would seem possible to obtain a suitable linearity of the order of 10% over an excursion .DELTA.F of 300 MHz and a good antenna matching over 400 MHz with certain types of antennas. The minimum range could then be lowered, at least theoretically to 1.5 m with a .DELTA.F of 300 MHz.
As a consequence of the quasi-theoretical limitations mentioned above, several techniques can be envisaged for producing a single-antenna radar:
The known technique of the radar known as "pseudo FM/CW" consists, for short range measurements, in chopping the transmitted signal according to sampling theory and in transmitting trains of waves whose duration must also be very short, for example less than 6.6 ns for a range to be measured of 1 m. It is still difficult to produce switches for microwave frequency signals as fast as this and this is a technological limitation for this technique. An FM/CW radioaltimeter has already been proposed in which the transmitter and the receiver function continuously using a common antenna. In this radioaltimeter the local signal of the radio frequency mixer of the receiver is obtained by reflection of the signal transmitted by the S.W.R. (Standing Wave Ratio) of the antenna. The construction is thus considerably simplified but to the detriment of the minimum measurable altitude, which can only reach 6 to 10 m, i.e. a value that is too high for the applications required for the radar according to the present invention. There is also known, particularly from French patent No. 2,541,465, a single-antenna FM/CW radar intended in particular for the measurement of short ranges and which raises exactly the same technical problem as in the present description: in a FM/CW radar equipment, if the transmitted signal and the reflected signal are duplexed on a common antenna, an interference signal due to coupling between the transmitter and the receiver is produced; this composite coupling signal results mainly in a partial reflection at the antenna which has a coefficient of reflection of finite and non-zero magnitude and in a leakage signal introduced by the duplexer circuit whose coefficient of directivity is technically limited. It follows that this composite radio frequency coupling signal, after demodulation in the input mixer of the receiver, creates, at the input of the audiofrequency amplifier of the receiver, interference signals which are partially situated in the useful pass band of the echo signal and with comparable amplitudes. This results in a limitation in the sensitivity of detection of a close object. In order to combat this lack of sensitivity, it is known that a static adjustment can be made which consists in adjusting the lengths of lines and possibly in adjusting a phase-shifter placed in one of these lines so that the phase of the local oscillation coincides with that of the main interference signals due to the S.W.R. of the antenna. The static adjustment is however insufficient in itself to achieve the required accuracy for range measurements in the order of 1.5 m to 3 m.
In the above mentioned French patent, an additional measurement, in order to improve the sensitivity of the radar, consists in introducing, after the input mixer of the reception channel, an audiofrequency amplifier having a second input which is connected, by means of a level modulator, to the frequency modulation signal generator of the transmitter, this level modulator having a control input sensitive to an adjustable d.c. voltage signal.